$slice (aggregation)

Definition定义

$slice

Returns a subset of an array.返回数组的子集。

$slice has one of two syntax forms:有两种语法形式之一：

The following syntax returns elements from either the start or end of the array:以下语法从数组的开头或结尾返回元素：

{ $slice: [ <array>, <n> ] }

The following syntax returns elements from the specified position in the array:以下语法从数组中的指定位置返回元素：

{ $slice: [ <array>, <position>, <n> ] }

Operand运算数	Description描述
<array>	Any valid expression as long as it resolves to an array.任何有效表达式，只要它解析为数组即可。
<position>	Optional. 可选。Any valid expression as long as it resolves to an integer.任何有效表达式，只要它解析为整数。 If positive, $slice determines the starting position from the start of the array. 如果为正值，$slice将从数组开始处确定起始位置。If <position> is greater than the number of elements, the $slice returns an empty array.如果<position>大于元素数，则$slice返回一个空数组。 If negative, $slice determines the starting position from the end of the array. 如果为负值，$slice将从数组末尾确定起始位置。If the absolute value of the <position> is greater than the number of elements, the starting position is the start of the array.如果<position>的绝对值大于元素数，则起始位置为数组的起始位置。
<n>	Any valid expression as long as it resolves to an integer. 任何有效表达式，只要它解析为整数。If <position> is specified, <n> must resolve to a positive integer.如果指定了<position>，则<n>必须解析为正整数。 If positive, $slice returns up to the first n elements in the array. 如果为正，$slice最多返回数组中的前n个元素。If the <position> is specified, $slice returns the first n elements starting from the position.如果指定了<position>，$slice将返回从该位置开始的前n个元素。 If negative, $slice returns up to the last n elements in the array. 如果为负，$slice将返回数组中的最后n个元素。n cannot resolve to a negative number if<position> is specified.如果指定了<position>，n不能解析为负数。

For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Behavior行为

{ $slice: [ [ 1, 2, 3 ], 1, 1 ] }

[ 2 ]

{ $slice: [ [ 1, 2, 3 ], -2 ] }

[ 2, 3 ]

{ $slice: [ [ 1, 2, 3 ], 15, 2 ] }

[  ]

{ $slice: [ [ 1, 2, 3 ], -15, 2 ] }

[ 1, 2 ]

Example示例

A collection named users contains the following documents:名为users的集合包含以下文档：

{ "_id" : 1, "name" : "dave123", favorites: [ "chocolate", "cake", "butter", "apples" ] }
{ "_id" : 2, "name" : "li", favorites: [ "apples", "pudding", "pie" ] }
{ "_id" : 3, "name" : "ahn", favorites: [ "pears", "pecans", "chocolate", "cherries" ] }
{ "_id" : 4, "name" : "ty", favorites: [ "ice cream" ] }

The following example returns at most the first three elements in the favorites array for each user:以下示例最多为每个用户返回favorites数组中的前三个元素：

db.users.aggregate([
   { $project: { name: 1, threeFavorites: { $slice: [ "$favorites", 3 ] } } }
])

The operation returns the following results:该操作返回以下结果：

{ "_id" : 1, "name" : "dave123", "threeFavorites" : [ "chocolate", "cake", "butter" ] }
{ "_id" : 2, "name" : "li", "threeFavorites" : [ "apples", "pudding", "pie" ] }
{ "_id" : 3, "name" : "ahn", "threeFavorites" : [ "pears", "pecans", "chocolate" ] }
{ "_id" : 4, "name" : "ty", "threeFavorites" : [ "ice cream" ] }